1. Field of the Invention
This invention relates to DC motor control systems and more particularly to control systems for web transport reel motors of the switching type.
2. Description of the Prior Art
DC motors operating in applications such as magnetic tape reel motors where moderate to large amounts of power are consumed are sometimes driven by a switching amplifier. The switching amplifier rapidly switches the motor drive to a low impedance voltage source.
A DC motor has a certain magnitude of motion response for a given low frequency drive signal applied thereto. For example, a drive signal varying slowly between plus and minus 10 volts might cause a given motor to vary in speed between plus and minus 500 R.P.M. If the frequency of the drive signal is increased while other factors remain constant a point will be reached where the peak motor speed will start to decrease below 500 R.P.M. The frequency of the input signal at which the maximum speed is down 3 decibels from the low frequency maximum speed is known as the cutoff frequency of the motor speed response. This cutoff frequency will typically be less than 100 Hz for a free running motor and will decrease as an inertial load is applied to the motor.
If the switching amplifier applies switching signals to the motor at a rate much faster than the motor cutoff frequency, for example, at least 10 times as fast, the motor response to a single short duration switching pulse will be negligible. However, the motor can be smoothly controlled by varying the average value of the switched signals by controlling their respective time durations. For example, an average 10 volt drive signal could be generated from a 20 volt power supply by switching the motor between 20 volt energization and no energization in a 50% duty cycle with a frequency at least 10 times the motor cutoff frequency. Since the drive circuit is either full on or full off it need dissipate no power.
To accomplish the same 10 volt drive from a 20 volt power supply, a linear drive circuit would have to dissipate power equal to 10 volts times the motor current. Multiple large, expensive power transistors as well as cooling fans might be required to dissipate this power.
The switching type of motor drive amplifier thus has a significant advantage from the standpoint of power dissipation. However, such circuits have disadvantages as well. For example, the motor energization command signals are typically continuously varying analog voltage or current signals that must be converted to switching signals having duty cycles proportional to the magnitudes of the analog voltages. Complex and expensive circuitry is required for this transformation of signal form. In addition, a motor drive power amplifier does not turn off instantaneously when commanded to do so. Because the transistors of a switching amplifier operate in a saturated condition, significant delays may occur between termination of an energization command and actual termination of motor drive current. If one polarity of switch turns on before the other turns completely off a short circuit will develop between the positive and negative power supply voltages and the resulting high currents can damage the switching transistors. A switching drive amplifier may thus require additional circuitry to prevent short circuits and expensive repairs when such circuitry fails.
In addition, the power supply provides low impedance voltage sources. As a result motor current can become quite high. Such currents can make stable control of a motor more difficult and can damage the power transistors if they become great enough.